Lets see...you said you're a Rookie, in an industry you know nothing about, and you've been hired by a very successful direct mail company...
I'd say write a letter as good as the one you used to sell this company on your services and you should be all set.
Seriously though, even the best copywriters don't hit the ball out of the park every time at bat, so unless you can sell something like Mike suggested - I don't think theres anything that could guarantee you success.